Pulse or digital communications – Repeaters – Testing
Patent
1990-04-13
1991-02-19
Tarcza, Thomas H.
Pulse or digital communications
Repeaters
Testing
329300, H04B 1500, H03D 310
Patent
active
049950525
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to transfer of data by means of direct sequence spread spectrum signals (DSSS) where the code is modulated on a carrier by means of MSK (Minimum Shift Keying) or related types of modulation, e.g. filtered MSK, and where the receiver does not have to recognize or retrieve the phase of the carrier. This is called incoherent reception.
MSK is a well known modulation technique. It is for instance described in the following articles: Carl Erik Sundberg: Continuous Phase Modulation. IEEE Communication Magazine, April 1986; S. Pasupathy: Minimum Shift Keying; A spectrally efficient Modulation. IEEE Communication Magazine, July 1979; F. Amoroso, J. A. Kivett: Simplified MSK Signaling Technique. IEEE Transactions on Communications, April 1977. The technique is efficient in the sense that the bandwidth which is required for transferring a certain amount of information per second is less than what for instance is required in connection with phase shift keying (PSK). In addition the MSK technique gives a greater possibility for operating with a constant envelope in a bandwidth limited channel than many other modulation techniques. These two characteristics make MSK an attractive form of modulation.
DSSS is a well known coding method within military communication where the purpose a.o. is to make the receiver less sensitive to disturbing signals. The method is for instance described in a book by R. C. Dixon: Spread Spectrum Systems, Wiley 1976 and it is based on the principle of coding every single data bit with a code which is known by the receiver. The code is usually modulated on the carrier with PSK, but MSK has also been used. The data are usually modulated with differential phase shift keying (DPSK).
Orthogonal signalling is a known modulation method which is described in a book by J. G. Proakis: Digital Communications, Mc Graw Hill 1983. This modulation method is based on the principle of letting a number of data bits (e.g. 1) be represented by a number of different (orthogonal) codes (e.g. 2). The receiver must be able to detect which of the orthogonal codes (e.g. 2) which was transmitted. When a code has been chosen to be the correct one, the receiver can determine the data bit which was transmitted. The method is commonly used for incoherent reception and in cases where the bandwidth in a channel is greatly limited.
Incoherent operation is used in receivers which cannot rely on phase information for demodulation of data, because such information is not available. This has e.g. been described in the mentioned book by J. G. Proakis. The signal in such a receiver is split in an inphase and a quadrature channel and the signal in each of these is squared, whereupon the two channels are added. It is impossible to use this method directly in connection with MSK.
From U.S. Pat. No. 4,583,048 there is known an MSK digital demodulator for burst communication. The object of the demodulator described is to undertake a rapid measurement of the phase difference between the incoming carrier and the local reference, so that coherent, conventional reception of data can take place. It is emphasized in the patent how important it is with burst communication systems to regain phase coherency rapidly, and it is pointed out that this cannot be obtained with conventional methods (PPL). The patent does, however, not mention anywhere how data demodulation is obtained. It appears that only the mutual amplitude of the four correlation peaks from each of the two paths are measured incoherently. These measurements are used to regain the phase so that the system can be brought to operate as a conventional coherent system for demodulation of data.
The purpose of the present invention is to provide a method for incoherent demodulation of a carrier of orthogonally coded DSSS signals which are modulated with MSK or related modulation types.
The main features of the invention are defined in the claims. In the present system the orthogonal codes are identical with the spreading code for the DSSS signals. Extraction
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Cain David
Forsvarets Forskningsinstitutt
Tarcza Thomas H.
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